What are the main problems hindering current AI development?
we don't really know what intelligence is.
we don't truly understand the best model of intelligence we have available (human intelligence) works.
we're trying to replicate human intelligence (to some extent) on hardware which is quite different from the hardware it runs on in reality.
the human brain (our best model of intelligence) is mostly a black-box to us, and it's difficult to probe/introspect its operation without killing the test subject.  This is, of course, unethical and illegal. So progress in understanding the brain is very slow. 


Combine those factors and you can understand why it's difficult to make progress in AI. In many ways, you can argue that we're shooting in the dark. Of course, we have made some progress, so we know we're getting some things right. But without a real comprehensive theory about how AI should/will work, we are reduced to a lot of trial and error and iteration to move forward.